Super Austenitic Stainless Steel Selection: 254SMO vs 904L vs Alloy 20 — How to Specify UNS S31254, N08904, and N08020 for Chloride and Acid Service
Super austenitic stainless steel is what buyers reach for when 316L corrodes but a nickel alloy is not yet affordable. Three grades dominate that gap — 254SMO (S31254), 904L (N08904), and Alloy 20 (N08020) — and they are routinely quoted against each other even though they solve different problems. Choosing the wrong one usually shows up as pitting in a chloride environment, or as stress corrosion cracking in a hot acid line, six months after commissioning.
The selection is not driven by a single “corrosion resistance” ranking. It is driven by which mechanism is attacking your equipment: chloride pitting, chloride stress corrosion cracking, or reducing-acid general corrosion. Each of these three grades is strong against one and weak against another.
The Three Grades Side by Side
| 254SMO (S31254 / 1.4547) | 904L (N08904 / 1.4539) | Alloy 20 (N08020 / 2.4660) | |
|---|---|---|---|
| Family | Super austenitic, 6% Mo | High-alloy austenitic, 4.5% Mo | Ni-Cr-Fe with Cu and Mo |
| Typical composition | 20Cr-18Ni-6Mo-0.2N-Cu | 20Cr-25Ni-4.5Mo-1.5Cu | 20Cr-35Ni-2.5Mo-3.5Cu-Nb |
| PREN | ~43 | ~34 | ~29 |
| Nickel content | ~18% | ~25% | ~35% |
| Strongest against | Chloride pitting and crevice corrosion in seawater | Hot sulphuric acid, general acid corrosion | Sulphuric acid with chlorides, mixed acids |
| Weakest against | Reducing acids — no Cu to help | Chloride pitting above ~60 °C | Chloride pitting — lowest PREN of the three |
PREN: What It Does and Does Not Tell You
PREN (Pitting Resistance Equivalent Number) is the most quoted and most misused number in stainless selection:
PREN = %Cr + 3.3 × %Mo + 16 × %N
It ranks resistance to chloride pitting. That is all it does.
- It says nothing about general corrosion in acids, where nickel and copper content matter more than chromium and molybdenum.
- It says nothing about stress corrosion cracking resistance, which is driven by nickel content and by the actual stress and temperature.
- It says nothing about crevice corrosion, which is more severe than pitting and is often what actually fails a flange or a gasket face.
A buyer who selects purely on PREN will pick 254SMO for everything, pay a premium, and still get attacked in a hot reducing acid where Alloy 20 would have been the correct and cheaper answer.
Selection by Corrosion Mechanism
Mechanism 1 — Chloride pitting and crevice corrosion (seawater, brine, cooling water)
This is 254SMO’s home ground. With a PREN around 43 and 6% molybdenum plus nitrogen, it resists pitting in chlorinated seawater at temperatures where 904L and 316L both pit. Where the duty is seawater at ambient to moderate temperature, and where crevices exist at flanges and gasket faces, 254SMO is the standard answer and the price premium over 904L is usually justified.
Choose this when: seawater cooling, desalination, offshore topside, chlorinated water, pulp mill bleach plant, flue gas desulphurisation with chlorides.
Mechanism 2 — Hot sulphuric acid and general acid corrosion
This is 904L’s territory. Its 25% nickel and 1.5% copper make it far more resistant to general corrosion in sulphuric acid than the PREN suggests. It is widely used in sulphuric acid service, phosphoric acid, and fertiliser plants, where the environment is acidic and reducing rather than chloride-pitting.
Choose this when: sulphuric acid at moderate concentration and temperature, phosphoric acid, acid pickling lines, fertiliser production, acetic and formic acid.
Do not choose it for hot chloride service — above roughly 60 °C in chlorides, 904L will pit, and its PREN of ~34 is only modestly better than 316L.
Mechanism 3 — Sulphuric acid with chlorides, and mixed acid streams
This is where Alloy 20 wins, and it is the grade most often overlooked. With 35% nickel, 3.5% copper, and niobium stabilisation, Alloy 20 handles sulphuric acid contaminated with chlorides — a combination that defeats 904L — and it resists both general corrosion and chloride pitting reasonably. Niobium stabilisation also makes it resistant to intergranular corrosion after welding, which matters for fabricated vessels and pipework that cannot be solution-annealed.
Choose this when: sulphuric acid with chloride contamination, mixed acid service, acid storage and handling equipment, pumps and valves in acid duty, welded fabrications in acid service.
Mechanical and Fabrication Differences
| 254SMO | 904L | Alloy 20 | |
|---|---|---|---|
| Yield strength | Highest (nitrogen-strengthened, ~300 MPa) | ~220 MPa | ~240 MPa |
| Weldability | Good, but needs matching filler and controlled heat input | Good, widely welded | Excellent — Nb-stabilised, tolerant of welding |
| Machinability | Difficult — work hardens, gummy | Moderate | Moderate; better than 254SMO |
| Formability | Lower — high strength means more springback | Good | Good |
| Availability | Narrower; plate, sheet, bar, tube from specialist mills | Good — widely stocked | Narrower — fewer mills, longer lead times |
| Relative cost | High (Mo + N) | Moderate–high (Ni) | High (Ni + Nb) |
Practical note on 254SMO: its high strength and work-hardening rate make it unforgiving in fabrication. Bending requires more force and more springback allowance than 316L; machining needs sharp tooling, low speed, and generous feed to get under the work-hardened skin. If your shop has never formed 254SMO, budget for trial pieces.
Forms and Standards to Specify
| Form | 254SMO | 904L | Alloy 20 |
|---|---|---|---|
| Plate / sheet | ASTM A240, EN 10028-7 | ASTM A240, EN 10028-7 | ASTM B463, EN 10095 |
| Bar | ASTM A276, A479 | ASTM A276, A479 | ASTM B473 |
| Seamless tube | ASTM A312 (S31254) | ASTM A312 (N08904) | ASTM B464 / B729 |
| Welded tube | ASTM A249, A269 | ASTM A249, A269 | ASTM B464 |
| Fittings / flanges | ASTM A403, A182 | ASTM A403, A182 | ASTM B366, B462 |
Note that Alloy 20 is a nickel alloy in ASTM terms, not a stainless steel — it is specified under B-series standards, not A-series. Quoting it against A240/A312 documents creates confusion at the mill and at goods-inspection. Specify the correct standard family.
Documentation and Verification
- EN 10204 3.1 certificate with heat traceability — mandatory at these price levels.
- PMI on receipt for every heat. These three grades are visually identical to 316L and to each other. A 316L delivered as 254SMO is a very expensive and very dangerous substitution, and it will not show up until the equipment fails.
- Intergranular corrosion test to ASTM A262 or EN ISO 3651-2, particularly for Alloy 20 and 904L fabrications that see acid service.
- Ferrite content for welded 254SMO — the nitrogen-bearing microstructure should be fully austenitic; sigma or ferrite phases reduce corrosion resistance.
- Surface condition: pickled and passivated, free of embedded iron and heat tint. In chloride service, a contaminated surface pits before the parent metal does.
RFQ Checklist
- Grade with UNS and EN/wrought equivalent stated: 254SMO (S31254 / 1.4547), 904L (N08904 / 1.4539), Alloy 20 (N08020 / 2.4660)
- Form and standard family: A240 for plate, A312 for tube, B463/B473/B464 for Alloy 20
- Dimensions and tolerances: thickness × width × length, or OD × wall × length
- Condition: solution annealed, pickled and passivated; delivery condition stated
- Service environment described in the RFQ: medium, concentration, temperature, chloride level, and whether the duty is continuous or cyclic
- Test requirements: 3.1 certificate, PMI, intergranular corrosion test, ferrite content, hydrostatic or eddy-current test for tube
- Fabrication intent: will it be welded, formed, or machined — this affects the condition and the filler recommendation
- Quantity, packing, destination port, Incoterm
FAQ
Is 254SMO always better than 904L? No. 254SMO wins on chloride pitting and crevice corrosion. In hot sulphuric acid, 904L’s nickel and copper content make it the better and often cheaper choice. The PREN ranking misleads here.
Can I use 904L in seawater? Only at low temperature and with care. Above roughly 60 °C in chlorinated seawater, 904L is prone to pitting and crevice attack. 254SMO is the correct grade for warm seawater.
Why specify Alloy 20 instead of 904L? When the acid contains chlorides, or when the fabrication will be welded and cannot be solution-annealed. Alloy 20’s copper and niobium handle both cases better.
Can these grades be substituted for each other in an emergency? Not without engineering review. They differ in pitting resistance, acid resistance, strength, and weldability. A substitution that survives the first month can fail in the second year.
Why Source Super Austenitic Grades from Yihang Metal
Yihang Metal supplies 254SMO (S31254), 904L (N08904), and Alloy 20 (N08020) in plate, sheet, bar, seamless and welded tube, and matching fittings, to ASTM A240, A276, A312, B463, B473, and B464, in solution-annealed, pickled, and passivated condition. Every heat ships with an EN 10204 3.1 certificate traceable to the piece, with PMI verification, intergranular corrosion testing, and ferrite content checks available on request. Standard production runs typically ship in 30–40 days for these grades.
Ready to specify your super austenitic grade? Send us the grade, form and standard, dimensions, the service medium with concentration, temperature and chloride level, and the test requirements, and we will confirm mill availability and lead time with a fixed-price quote and test certificates. Email contact@yihangmetal.com or use the quote form on this page — our team replies within one business day.